Some airline reservation systems have web interfaces (or web services) that permit a travelers or travel agencies to request seats for travel on airline flights. In building an itinerary for a trip, travelers or travel agencies can also request reservations from several other travel provider systems such as car rental agencies and hotels. However, in certain circumstances, airline flights may be rescheduled or cancelled, which can be very inconvenient for travelers not only because the flight was changed but also because other travel services are affected by the changed flight. (e.g., the other travel services are unnecessary because a traveler's flight was cancelled).